Method of fabricating thin graphite reinforced composites of uniform thickness

ABSTRACT

A method of making high strength, low thermal expansion thin composite structures of uniform thickness is disclosed. A plurality of resin impregnated graphite fiber sheets are laminated together to provide composite preforms. A plurality of the preforms are then bonded together in a pseudoisotropic arrangement between parallel plates. The resulting structures are especially useful as substrates for flat or slightly contoured mirrors.

ilnited States Hertz atent 1 1 [451 Dec. 13,1973

[ METHOD or FABRHCATHNG THEN GRAPHITE REINFORCED COMPUSHTES or UNIFORMTHICKNESS [75] Inventor: Julius Hertz, San Diego, Calif.

[73] Assignee: General Dynamics Corporation, San

Diego, Calif.

22 Filed: Dec. '22, 1971 21 Appl. No.: 210,694

US. CL... 156/309, 156/161, 156/163,

52 15 6/299,156/306, 161/55, l61/57,161/58, 161/141 [51 1111. C1. C09j5/00, 1332b 5/12 [58] Field 01" Search 156/161, 163, 299,

[56]- 5 5 References Cited UNITED STATES PATENTS 2,901,455 8/1959 Jurras161/58 3,432,295 3/1969 Frank et a1. 75/206 3,466,219 9/1969 Schwartz161/57 3,711,934 1/1913 Zorowski et a1. 29/445 Primary Examiner-AlfredL. Leavitt Assistant Examiner-Robert A. Dawson Attorney-John R. DuncanABSTRAC A method of making high strength, low thermal expansion thincomposite structures of uniform thickness is disclosed. A plurality ofresin impregnated graphite fiber sheets are laminated together toprovide composite preforms. A plurality of the preforms are then bondedtogether in a pseudoisotropic arrangement between parallel plates. Theresulting structures are especially useful as substrates for flat orslightly contoured mirrors.

18 Claims, 2 Drawing Figures lMPREGNATE GRAPHE'TE i SHEETS 1 men-arm emuenerenm sunrnees LAWNATE enerenme 111 s Pseunmsnrmme meanest/1am FORMREFLECTWE SURFAQE PAIENIEBDEC 18 mm 3. 779.851

IMPREGNATE GRAPHITE FIBER SHEETS WITH RESIN LAMINATE SETS OF SHEETS TO 2PRODUCE PREFORIVIS CURE RESIN /3 LIGHTLY SAND PREFORM SURFACES LAMINATEPREFORMS IN PSEUDOISOTROPIC ARRANGEMENT FORM REFLECTIVE SURFACE FIG.|

INVENTOR. JULIUS HERTZ METHOD OF FABRICATING THIN GRAPHITE REINFORCEDCOMPOSITES OF UNIFORM THICKNESS BACKGROUND OF THE INVENTION Thisinvention relates to reinforced plastic composite structures and,'morespecifically, to methods of making thin composite sheet or platestructures of improved flatness and uniform thickness.

Recently, reinforced plastic composite structures have been developedfor use in a wide variety of structural applications. These materialshavea number of advantages over metal, ceramic or plastic structures.Composite materials, which may, for example, comprise graphite, carbonor glass fibers imbedded in a plastic matrix, generally have very highstrength-toweight ratios, and very low thermal expansioncharacteristics. These characteristics are highly desirable for severaloptical applications, among others. Very flat mirrors or preciselycontoured mirrors are required in a number of optical instruments-Thelight weight and high stiffness of these materials make them highlydesirable in portable optical devices and in optical devices to be usedin outer space.

These materials have not, however, been entirely successful in certainapplications requiring very flat or precisely contoured thin plates ofvery uniform thickness. conventionally, composite materials are preparedby impregnating a plurality of sheets of the reinforcing fiber materialwith the resin, then shaping the material while curing the resin.Thermal stresses are set up in the material during the molding andcuring steps which result in warping of the resulting composite plate.These stresses also change with temperature changes, so that even if aplate can be made flat at one temperature, it is likely to distort atother temperatures.

While it is possible to reduce distortion of flat composite plates byeither making them thicker or laminating them to other structures, theincreased thickness is undesirable for many applications and adds to thestructure weight. Many optical devices, such as Baez or grazingincidence devices, may be dimensionally limited in substrate thicknessto a thin plate. A thin sheet or plate also is superior in that there isa minimum temperature gradient across the thickness of the plate. Thisminimizes distortions due to thermal bending.

Attempts have been made to produce relatively flat small specimens bycutting the best areas from larger panels. However, this technique isvery wasteful and produces only very small plates. Smoothing a surfaceby sanding has little utility, since the plate cannot be sanded beyondremoval of surface glaze without reaching the imbedded fibers. Sandinginto the fibers destroys plate symmetry, produces a non-uniform surfaceand increases the possibility of thermal distortion. Similar problems,of course, occur in the fabrication of precisely contoured spherical orparabolic mirrors.

Thus, there is a continuing need for an improved process formanufacturing very flat and precisely contoured thin plates fromreinforced plastic composite materials.

SUMMARY OF THE INVENTION An object, therefore, of this invention is toprovide a composite material forming process overcoming the above-notedproblems.

Another object of this invention invention to provide a method ofmanufacturing very flat or precisely contoured thin plates fromcomposite materials.

Still another object of this invnention is to provide a method ofmanufacturing thin plates of highly uniform thickness from compositematerials.

The above objects, and other, are accomplished in accordance with thisinvention by a method which comprises the steps of impregnating aplurality of thin sheets of a graphite fiberous reinforcing materialwith a synthetic resin, forming a number of preforms by laying up fromtwo to 10 sheets and curing the resin while the sheets are held in asubstantially flat arrangement, then bonding a plurality of theresulting preforms together 'in a pseudoisotropic arrangement to formthe final flat plate.

DETAILED DESCRIPTION OF THE INVENTION While any suitable number ofsheets of the graphite reinforcing material may be laminated together toproduce the preforms, preferably from two to 10 are used. Best resultsare generally obtained with from two to 10 sheets because of theaccurate fiber alignment possible with this small number of sheets andthe ease with which the resulting preforms can be slightly deformedduring subsequent bonding operations. While these sheets may belaminated in any suitable orientation, for best results either apseudoisotropic or unidirectional arrangement is preferred. All fiberousreinforcing material whether a fiber mat, a woven sheet or any other,has a direction of maximum strength across the sheet surface. Thus allfiberous sheets are directionally orientated to some extent.

In a unidirectional arrangement, all sheets have their fibers running ina single direction, or 0." ln a pseudoisotropic arrangement, the maximumstrength direction of each fiber sheet is oriented with respect to theothers to balance the preform strength and the sheets are balanced aboveand below a central plane through the laminate parallel to the laminatefaces. For example, in a four-sheet pseudoisotropic preform, the sheetswould be laminated with the two maximum strength lines at right anglesto each other; that is a O/90/90/0 lay-up. With six sheets, the angularrelationship might be 0/+60/60/60/+60/0 and with eight sheets,0/+45/-45/90/90I 45/+45/0, etc. These preforms are likely to be slightlynon-flat after the impregnating resin has cured. However, since thesepreforms are thin, they require extremely low loads for flattening.Thus, pseudoisotropic bonding of a number of preforms (eitherunidirectional or pseudoisotropic preforms) has been found capable ofproducing an extremely flat plate from a number of randomly near-flatpreforms. Similarly, a plate having a precisely contoured spherical orparabolic surface may be formed from a plurality of preforms whichindividually are formed in nearly the desired shape, then are laminatedagainst a precisely shaped surface.

Any suitable number of preforms may be bonded together to give a finalthin flat or precisely contoured plate of a selected thickness. Ingeneral it is preferred that from two to l0 preforms be used. An optimumcombination of plate stiffness and strength, together with lowest weightand thickness is generally obtained with plates having thicknesses offrom about 0.05 to 0.2-5.in ch. Best results are generally obtained withthe maximum number'of layers of the starting sheets and of theintermediate preforms, since this increases the isotropy (uniformity ofproperties) of the plate. The starting sheets are generally available inthe 0.001 to 0.01 inch thickness range, so that preferred plates maycontain from about 12 to I layers of these sheets. To obtain very flatplates, it is strongly preferred that a fully pseudoisotropic assemblybe used. Both the directional orientation of the preforms should bebalanced and the number and orientation of preforms above and below acentral plane through the plate parallel to the plate surface should bebalanced.

The graphite fiber sheets may be impregnated with, and bonded togetherby, any suitable resin during preform manufacture. Typical syntheticresins include polyolefins such as polyethylene an polypropylene; vinyland vinylidene polymers such as polystyrene and polyvinylacetate;fluorocarbons such as polytetrafluorethylene and polyvinyl fluoride;polyamides such as polycaprolactam; polyimides; polyurethanes;polysulfides; polysulfones; polycarbonates; phenolic resins such asphenol-formaldehyde resins; polyesters; epoxy resins; silicone resins;alkyd and alkyl resins; polyquinoxalines; polyphenylquinozalines;polyimidazoquinoxalines; and mixtures and copolymers thereof.Impregnation may be accomplished by any suitable techinique, such assolution dipping, spraying, hot-melt coating, etc. The resin may becured by any suitable technique. Resins which are thermally orcatalytically cured are preferred, since they produce plates with verylittle residual solvent or other ingredients which might out-gas if theplate is used in a vacuum environment and they are easily cured in openor closed molds. Best results have been obtained with epoxy andpolyimide resins. These produce a strong, rigid plate with excellentthermal stability. By properly selecting fiber types and resin matrices,it is possible to selectively vary the coefficient of expansion of theplates between about 0.6 X in./in/F and +20 X 10' in./in.F.

Any suitable graphite fibers arranged in sheet form may be used as thereinforcing element. If desired, other reinforcing fibers may be addedto enhance specific desired physical properties of the plates. Typicalother fibers include boron, glass, asbestos and quartz fibers. The fibersheets may have any suitable structure. Typically, sheets may beprovided as mats of chopped fibers, woven fiber cloth-like sheets, etc.Generally, the fiber sheets should have thicknesses in the range ofabout 0.001 to 0.005 inch for best results. While individual fibers mayhave any suitable dimensions, generally diameters of from about 8 to 50microns and lengths of from about 0.5 inch to continuous are preferred.Best results are generally obtained with socalled prepreg materialswhich consist of a single layer of parallel fibers or filaments coatedwith a resin or plastic matrix material, then partially curing theresin. Typically, an epoxy resin may be applied then partially cured tothe stage in which it is tacky, then cured to the hard stage afterpreform assembly. The prepreg sheets are handled in the form of acontinuous tape or web, or as cut sheets, spaced apart with intersheetshaving anti-stick surfaces.

After impregnation with the selected resin, the fiber sheets areassembled in the desired orientation and pressed between flat platesduring the resin curve period. Typically, with epoxy resins, curing fora minimum of about 2 hours at a temperature of about 375 F give bestresults.

After the desired number of preforms are produced, they are bondedtogether in the selected pseudoisotropic arrangement to make the desiredplate. Any suitable bonding technique may be used. Typically, adhesivebonding with room temperature bonding epoxics, such as EC-22l6(available from 3M Co.) may be used. Where adhesive bonding is used, itis preferred that the adhesive layer have a thickness less than about0.002 inch. Plates having many thin preforms with very thin adhesiveinter-layers are less subject to the saddle" effect (transversedistortion) when distorted in the longitudinal direction. Duringadhesive curve, the assembled plate is preferably pressed between flatplates at a pressure of about 50 psig and temperature of about 60 to F.Accurately controlled plate thickness is achieved by inserting metalstops or shims of accurate thickness between the pressing platesadjacent to the preform assembly.

While not always necessary, it is generally preferred that the preformsurfaces be very lightly sanded to remove any surface glaze before theyare bonded together to produce the composite plates. This sanding shouldnot penetrate to the fibers within the preforms.

Where a finished flat plate is to be used as a mirror substrate, thefinal step is the formation of a suitably reflective surface coating.Any suitable coating or replication technique may be used. Directpolishing of the plate surface may provide a reflective surface suitablefor some uses. However, it is preferred that a reflective surface layerbe used for most applications, especially in the ultraviolet and/orX-ray ranges. Any surface layer suitable for a desired application maybe used. Typically, a thin layer of low-expansion material, such asfused silica may be bonded to the plate, then ground and polished. Areflective metal coating, such as nickel, may then be applied, such asby vacuum evaporation. Or, a replication technique may be used,typically one in which a master optical surface is coated successivelywith evaporated silver, silicon monoxide, aluminum and silicon monoxide;the free surface of this assembly is bonded to a composite plate, thenthe assembly is parted from the master and the silver is dissolved away,leaving the overcoated aluminum surface exposed for high reflectance.

If desired, a reflective surface may be applied during manufacture ofthe composite plate. Typically, a master surface is plasma-sprayed withlow-expansion material, then covered with the lay-up of compositematerial and cured. After parting from the master, the plasmasprayedcoating is ground and polished. Alternatively, a precured compositeplate may be plasma-sprayed with a low-expansion material, then groundand polished.

BRIEF DESCRIPTION OF THE DRAWING Details of the invention will befurther understood upon reference to the drawing, wherein:

FIG. 1 is a block diagram flow sheet for the composite platemanufacturing process of this invention; and

FIG. 2 is a schematic exploded view of plate made by the process of theinvention.

DETAILED DESCRIPTION OF THE DRAWING Referring now to FIG. 1, there isseen a flow sheet illustrating a preferred process of making a typicalproduct, namely a thin, flat mirror of uniform thickness.

As discussed above, Step 1 consists of impregnating a plurality ofsheets made up of primarily graphite fibers with a suitable syntheticresin. Next, in Step 2, groups of from two-eight of these impregnatedsheets are bonded together in flat molds to form preforms. Preferably,the sheets are bonded in a pseudoisotropic arrangement. The resin isthen cured as shown in Step 3 while the preforms are held flat.

While not always necessary, it is generally preferred that the surfacesof the preforms be very lightly sanded, as shown in Step 4, to removethe surface glaze, before bonding them together in Step 5.

The number of preforms necessary to produce a final plate of a selectedthickness are then bonded together in a pseudoisotropic arrangement asillustrated in Step 5. While individual preforms are likely to beslightly non-flat, these distortions have been found to balance out whena number of preforms are combined, so that the resulting plate is veryflat and has very uniform thickness.

Upon completion of Step5, the plate is ready for use in any applicationrequiring flat plates of high strength and low thermal expansion ordistortion characteristics.

A preferred application for the flat composite plates produced by thisprocess is in flat or accurately contoured mirrors for use in opticalinstruments, optical devices for use in outer space, etc. Thus, apreferred final step, Step 6, involves the formation of a highlyreflective surface on the plate. This reflective surface may be formedin any suitable manner, as discussed above.

FIG. 2 shows an exploded view of a composite plate mirror as produced byStep 5 shown in FIG. 1.

The basic composite plate, generally designated 10, is made up ofpreforms 11-15, bonded together by a suitable adhesive (not shown). Eachpreform 11-15 has a direction of maximum fiber strength across itssurface, illustrated by arrows 16-20. These preforms 11-15 are bondedtogether in a pseudoisotropic arrangement to produce a plate with anoptimum balance of physical characteristics in all directions. Thus, inthe lay-up illustrated, preforms 11-15 are arranged with strengthdirections at 0, +45, 90, 45 and 0 as illustrated by arrows 16-20.

-After plate 10 is completed, a reflective layer 21 is formed on theupper surface of plate by any suitable technique. Typically, this layer21 may comprise a 0.010 to 0.030 layer of fused silica which is ground,polished and coated with a thin reflective nickel coating.

DESCRIPTION OF PREFERRED EMBODIMENTS Details of several preferredembodiments of the process of this invention will be further understoodupon reference to the following examples. All parts and percentages areby weight unless otherwise indicated.

from Union Carbide, is cut into about 8 inch diameter disks. This sheetmaterial contains sufficient fiber content to give a cured ply thicknessof about 0.002 inch. Preforms are prepared by stacking eight of thedisks with a unidirectional fiber orientation, then curing the v theproper thickness around the stack to control final plate thickness. Thebonded composite plate is cured for 24 hours at room temperature, thenfor 2 hours at F. A layer of fused silica having a thickness of about0.03 inch is bonded to the composite plate. Finally, the fused silica isground optically flat and a thin reflective coating of nickel is appliedby vacuum evaporation. The resulting mirror is especially suitable forX-ray optical devices in a vacuum.

EXAMPLE 11 A prepreg sheet as described in Example 1 is subdivided intoa number of preform sets of six 10 inch by 12 inch rectangular sheets,the plies in each preform having the following order of fiberorientation: 0/+45/-45/+45/0. The sheets making up each preform are thusstacked, then the preforms are cured for about 2 hours at about 350 Funder vacuum augmented autoclave pressure. Four preforms are then bondedtogether with Shell Chemical Corporations Epon 934 filled epoxy adhesiveto produce a final composite plate having three bondlines, each lessthan 0.002 inch thick. Bonding is done between ground steel plateshaving a flatness tolerance of about 0.0005, using shims to set thefinal plate thickness at 0.054 inch. The plate is then cured for 24hours at room temperature followed by about 2 hours at about 150 F. Anabout 0.03 inchthick layer of fused silica is bonded to the compositeplate. After optically grinding the silica layer, thin coatings ofsilicon monoxide, aluminum and silicon monoxide are applied by vacuumevaporation. The resulting mirror is suitable for use in infrared orvisible light regions.

EXAMPLE Ill The prepreg to be used is Morganites Type 1, highmodulusgraphite fiber impregnated with Fiberetes C-904 epoxy resin formulation,where the fiber has been spread to a thickness which will give a curedsheet thickness of about 0.0005 inch. Four pseudoisotropic preforms areprepared where each preform is made up of eight 10 inch by 12 inchrectangular sheets having the following fiber orientation: .qliti #91919! :ilfli/Qfiashmsbam ss es. for a minimum of 2 hours at about 375?under vacuum augmented autoclave pressure. The four preforms are thenbonded together with Crests 7343 polyurethane adhesive to get a finalplate having three bondlines, each less tha about 0.002 inch thick. Thepreforms are bonded between very flat steel plates, using shims toassure a final composite plate thickness of about0. 166 inch. The bondsare then cured for at least about 12 hours at room temperature. A castepoxy resin sheet having a thickness of about 0.03 inch is bonded tothe'composite plate. The epoxy sheet is ground optically flat, thencoated with thin layers of silicon monoxide, aluminum and siliconmonoxide. The resulting mirror is highly suitable for use in theinfrared region.

EXAMPLE IV A prepreg is prepared consisting of Courtlands HT-S highstrength graphite fibers impregnated with Plockton 951 polyester resinfrom Allied Chemical. The fibers are arranged to give a cured sheetthickness of about 0.007 inch. Two inch by 12 inch rectangularpseudoisotropic preforms are prepared, each made up of six plies withthe following fiber orientation: O/ l60/60/6 0 /1Z0]0jach preform iscured in a vacuum autoclave for at least about 2 hours at about 325 F.The preforms are bonded together between flat plates using Eastman HE-97polyester resin from Eastman Chemical Products. Shims determined thefinal composite plate thickness at 0.084 inch. The bond is cured for atleast about 4 hours at room temperature. A 0.03 inch layer of epoxyresin is formed on one surface of the composite plate and cured. Thelayer is then optically ground and overcoated with thin layers ofsilicon monoxide, aluminum and silicon monoxide by vacuum deposition. Anexcellent mirror for infrared use results.

EXAMPLE V A prepreg sheet is prepared by impregnating Hercules Type Agraphite fiber with Monsantos SC-l008 phenolic resin, with the fibersspread to give a cured sheet thickness of about 0.005 inch. Fourpseudoisotropic preforms are laid up on ground steel spherical molds.Each preform is made up of 14 plies having the following order 7 offiber orientation: 0/+45/+60/90/60/-45/0/0/45/45- /-60/90/+60/+45/0.Each preform is cured for at least about 2 hours at about 350 F undervacuum bag presure. The preforms are then bonded together withLefkowelds Type 109 epoxy adhesive to produce a panel with bondlinethicknesses of less than about 0.0002 inch. Bonding is accomplishedbetween two matching ground steel spherical molds having a contourtolerance of about 00005 inch, using shims to produce a final panelthickness of about 0.216 inch. The bonds are then cured for at leastabout 1 hour at about 150 F. A layer of nickel is then chemically platedon the concave panel surface, then processed to a highly accuratespherical configuration. The spherical mirror which results is highlyaccurate and suitable for X-ray use.

EXAMPLE VI A prepreg sheet is prepared using Whittaker- Morgans Modmor Ihigh-modulus graphite fibers impregnated with Monsantos RS-6234polyimide resin. The fibers are arranged so as to give a cured sheetthickness of about 0.005 inch. Two preforms are prepared, each of whichis made up of eight sheets having the 7 following fiber orientation:0/+45/90/fl 5- /45/90/+45/0. Each preform is cured for at least about 2hours at 350 F under vacuum-augmented autoclave pressure. The twopreforms are bonded together with Armstrongs A-4 aluminum-filled epoxy 6adhesive. The sheets are bonded between flat steel plates using shims togive a final composite plate thickness of about 0.082 inch, with anabout 0.002 inch bondline.

After curing the plate for about l2 hours at room temperature, a thinCER-VIT sheet is bonded to the plate. The CER-VIT sheet is groundoptically flat and a thin coating of reflective nickel is applied byvacuum evaporation. This highly fiat mirror is useful in X-ray devicesin a vacuum.

EXAMPLE VI] A prepreg material is prepared by impregnating a sheet ofwoven graphite fiber with a polyimide resin. The sheet has a curedthickness of about 0.002 inch.

The sheet is cut into a number of 6 inch diameter disks. While thefibers are woven, the disks do have greatest strength in one direction.Five preforms are prepared by stacking eight of the disks with theprimary strength direction oriented as follows: 0/+45/45/90/45/+45/0.The preforms are then cured at 400 F for 3 hoursunder pressure against a highly accurate spherically curved steelsurface. The preforms are then coated with EC-22 l 6, a filledepoxypolyimide adhesive from the 3M Company. Five preforms are stackedon the spherically curved steel surface with the primary strengthdirection of the top sheet in each preform oriented as follows:0/+45/90/45/0. A complementary spherically curved steel surface is thenpressed against the stack and the adhesive is cured for 24 hours at roomtemperature, then for 2 hours at F. The resulting plate is found to havea highly accurate spherical surface and to have very uniform thickness.

Although specific materials, and process variables are specified in theabove description of preferred embodiments, these may be varied asdescribed above with similar results. In addition, other ingredients maybe included in the fiber sheets, impregnating resin, preform bondingagent, surface coatings, etc. in order to modify or enhance desiredproperties of the system.

Various modifications, ramifications and applications of the presentinvention will occur to those skilled in the art upon reading thisspecification. These are intended to be included within the scope ofthis invention, as defined in the appended claims.

I claim:

1. A method of preparing composite structures of highly uniformthickness which comprise the steps of:

providing a plurality of thin sheets each comprising unidirectionallyoriented graphite fibers impregnated with a synthetic resin:

preparing a plurality of preforms by assembling a plurality of stackseach having from two to 20 sheets arranged in a selected firstorientation;

pressing each stack against a shaping surface while curing said resin;

assembling a plurality of said preforms in a stack with adhesiveinterlayers therebetween, said preforms arranged in a selected secondorientation;

at least one of said first and second orientations beingpseudoisotropic, and

bonding said preforms together while pressing said preforms between apair of parallel surfaces, each of which has substantially the sameconfiguration as said first surface, whereby a plate of highly uniformthickness is produced.

2. The method according to claim 1 wherein said first orientation ispseudoisotropic.

3. The method according to claim 1 wherein said second orientation ispseudoisotropic.

4. The method according to claim 1 wherein both said first and saidsecond orientations are pseudoisotropic.

5. The method according to claim 1 wherein all of said surfaces aresubstantially flat, whereby the plate produced is extremely flat.

6. The method according to claim 1 wherein from 2 to sheets areassembled to make each preform and form 2 to 10 preforms are assembledto make said plate.

7. The method according to claim 6 wherein said plate is formed to athickness of from about 0.05 inch to about 0.25 inch.

8. The method according to claim 1 wherein each sheet has athickness offrom about 0.001 inch to about 0.0] inch and each plate contains fromabout 12 sheets to about 100 sheets.

9. The method according to claim 1 including the further step of forminga highly reflective layer on at least one plate surface.

10. The method according to claim 1 wherein shims are placed betweensaid parallel surfaces adjacent to the preforms to maintain desiredplate and adhesive bondline thicknesses.

1 l. A method of preparing thin, flat composite plates which comprisesthe steps of:

a. providing a plurality of sheets each comprising unidirectionallyoriented graphite fibers impregnated with a synthetic resin;

b. stacking from two to 10 of said sheets on a first substantially flatplate surface in a selected first fiber orientation;

c. curing said resin to produce a rigid preform;

d. repeating steps (b) and (c) to produce at least one additionalsubstantially identical preform;

e. stacking from two to 10 of said preforms in a selected second fiberorientation on a second substantially flat shaping surface with adhesiveinterlayers between said preforms;

at least one of said first and second orientations beingpseudoisotropic; and

curing said adhesive while pressing said stack with a thirdsubstantially flat shaping surface which is maintained substantiallyparallel to said second surface and space a selected distance therefrom,whereby a highly flat and uniformly thick composite plate is produced.

12. The method according to claim 11 wherein said first orientation ispseudoisotropic.

13. The method according to claim 11 wherein said second orientation ispseudoisotropic.

14. The method according to claim 11 wherein said first and secondorientations are pseudoisotropic.

15. The method according to claim 11 wherein said plate is formed to athickness of from about 0.05 inch to about 0.25 inch.

16. The method according to claim 11 wherein each sheet has a thicknessof from about 0.001 inch to about 0.01 inch and each plate contains fromabout 12 sheets to about about sheets.

17. The method according to claim 11 including the further step offorming a highly reflective layer on at least one plate surface. I

18. The method according to claim 11 wherein said preforms are stackedso that fiber orientations are substantially identical on each side of aplane through the center of said plate parallel to the plate surfaces.

2. The method according to claim 1 wherein said first orientation ispseudoisotropic.
 3. The method according to claim 1 wherein said secondorientation is pseudoisotropic.
 4. The method according to claim 1wherein both said first and said second orientations arepseudoisotropic.
 5. The method according to claim 1 wherein all of saidsurfaces are substantially flat, whereby the plate produced is extremelyflat.
 6. The method according to claim 1 wherein from 2 to 10 sheets areassembled to make each preform and form 2 to 10 preforms are assembledto make said plate.
 7. The method according to claim 6 wherein saidplate is formed to a thickness of from about 0.05 inch to about 0.25inch.
 8. The method according to claim 1 wherein each sheet has athickness of from about 0.001 inch to about 0.01 inch and each platecontains from about 12 sheets to about 100 sheets.
 9. The methodaccording to claim 1 including the further step of forming a highlyreflective layer on at least one plate surface.
 10. The method accordingto claim 1 wherein shims are placed between said parallel surfacesadjacent to the preforms to maintain desired plate and adhesive bondlinethicknesses.
 11. A method of preparing thin, flat composite plates whichcomprises the steps of: a. providing a plurality of sheets eachcomprising unidirectionally oriented graphite fibers impregnated with asynthetic resin; b. stacking from two to 10 of said sheets on a firstsubstantially flat plate surface in a selected first fiber orientation;c. curing said resin to produce a rigid preform; d. repeating steps (b)and (c) to produce at least one additional substantially identicalpreform; e. stacking from two to 10 of said preforms in a selectedsecond fiber orientation on a second substantially flat shaping surfacewith adhesive interlayers between said preforms; f. at least one of saidfirst and second orientations being pseudoisotropic; and g. curing saidadhesive while pressing said stack with a third substantially flatshaping surface which is maintained substantially parallel to saidsecond surface and space a selected distance therefrom, whereby a highlyflat and uniformly thick composite plate is produced.
 12. The methodaccording to claim 11 wherein said first orientation is pseudoisotropic.13. The method according to claim 11 wherein said second orientation ispseudoisotropic.
 14. The method according to claim 11 wherein said firstand second orientations are pseudoisotropic.
 15. The method according toclaim 11 wherein said plate is formed to a thickness of from about 0.05inch to about 0.25 inch.
 16. The method according to claim 11 whereineach sheet has a thickness of from about 0.001 inch to about 0.01 inchand each plate contains from about 12 sheets to about about 100 sheets.17. The method according to claim 11 including the further step offorming a highly reflective layer on at least one plate surface.
 18. Themethod according to claim 11 wherein said preforms are stacked so thatfiber orientations are substantially identical on each side of a planethrough the center of said plate parallel to the plate surfaces.